The present invention relates generally to apparatus for treating bulk materials such as foundry sand and more particularly to a drum mixer which comprises a drum rotating about a horizontal or substantially horizontal axis with at least one chamber being formed within the drum and with the drum being rotated at such a speed that the bulk material contained therein is carried from an accumulated mass in the lower region of the drum upwardly by centrifugal force into the upper region of the drum where it is stripped off from the inner wall of the drum by a fixed deflection device forming the bulk material into a downwardly falling jet.
From the prior art, particularly German patent application No. H 13.467, there is known a machine of the aforementioned type wherein a falling jet produced by a deflection device is sprayed with an aggregate, e.g. a liquid binder. The movement of the deflection device operates to prevent formation of a crust upon the latter, but it has been found to be not possible with this device to prevent the formation of lumps in the bulk material or to remove lumps formed therein. In such prior art devices it has been found that the distribution of the material by centrifugal force is virtually nonexistent or existent only to a minor extent so that it is not possible to admix secondary components.
A further mechanism or mixing machine known in the prior art, specifically DT-AS No. 2003201, involves a rotating container inclined relative to the horizontal plane carrying close to its bottom area a curved baffle plate which extends from the inner wall to the outer circumference of a discharge opening in the bottom of the rotating container, and at least one rotating tool engaging the material and surrounded by the latter.
Since a machine of this type does not produce a fine falling jet, it is not possible to introduce binders so that the grains will be uniformly enveloped by the latter. A stirring tool dipping into the sand will not act as a fan blower and therefore will have only a moderate lump-dispersing effect. Furthermore, high power consumption is required so that the temperature of the bulk material will be unnecessarily increased thereby resulting in harmful effects. A homogeneous distribution of the fine portions of old sand and of newly added materials, such as bentonite, coal dust, etc. is only possible to a limited extent since the bulk material is not brought into a loosened state in the form of a falling jet.
Accordingly, it is an object of the present invention to provide a drum mixer of the aforementioned type which will enable optimum and rapid preparation and treatment of foundry sands while avoiding the aforementioned inconveniences where the material to be prepared is highly permeable to gas without subsequent treatment.